Those in the wide ranging materials joining industries have recognized the benefits of friction stir welding (FSW) since its invention, only to be precluded from widespread application due to a number of factors. One such long-recognized need has been that of providing a system that monitors and controls the load placed upon a FSW tool in the direction of travel of the tool thereby improving both the aesthetic and mechanical properties of a weld. This need is particularly prevalent in joining members having complex curvatures and variable thicknesses.
FSW is a relatively simple method of solid phase welding developed by The Welding Institute in the early 1990's. The process utilizes a specially shaped nonconsumable cylindrical tool with a profiled probe, often threaded, extending from a shoulder of the tool. The tool is rotated and plunged into a joint formed by abutting edges of the workpieces that are to be joined until a surface of the shoulder contacts the surface of the workpieces. The rotating tool plasticizes a region of the workpieces around the probe and beneath the shoulder. The tool is then advanced along the joint. The rotation of the tool develops frictional heating of the workpieces and the tool forces plasticized workpiece material from the leading edge of the tool to the rear of the tool, while the shoulder confines the plasticized material from above and the plasticized material consolidates and cools to form a high quality weld.
The FSW tool is generally formed as a cylindrical piece with a shoulder face that meets a probe that projects from the shoulder face at a right angle, as illustrated in U.S. Pat. Nos. 5,460,317 and 6,029,879. In some instances, the probe actually moves in a perpendicular direction in an aperture formed in the face of the shoulder, as illustrated in U.S. Pat. Nos. 5,611,469; 5,697,544; and 6,053,391. The face of the shoulder may be formed with an upward dome that is perpendicular to the probe, as illustrated in U.S. Pat. Nos. 5,611,479; 5,697,544; and 6,053,391. The dome region and an unobstructed shoulder face to probe interface are considered essential for the proper frictional heating of the workpiece material. The dome region serves to constrain plasticized material for consolidation at the trailing edge of the FSW tool so as to prevent it from extruding out from under the sides of the tool.
Since FSW is a solid-state process, meaning there is no melting of the materials, many of the problems associated with other fusion welding methods are avoided, including porosity, solidification cracking, shrinkage, and difficulties in weld pool positioning and control. Additionally, FSW minimizes distortion and residual stresses. Further, since filler materials are not used in FSW, issues associated with chemical segregation are avoided. Still further, FSW has enabled the welding of a wide range of alloys that were previously unweldable. Another advantage of FSW is that it does not have many of the hazards associated with other welding means such as welding fumes, radiation, high voltage, liquid metals, or arcing. Additionally, FSW generally has only three process variables to control (rotation speed, travel speed, and pressure), whereas fusion welding often has at least twice the number of process variables (purge gas, voltage, amperage, wire feed speed, travel speed, shield gas, and arc gap, just to name a few). Perhaps most importantly, the crushing, stirring, and forging of the plasticized material by the FSW tool produces a weld that is more reliable than conventional welds and maintains material properties more closely to those of the workpiece properties, often resulting in twice the fatigue resistance found in fusion welds.
Despite all the advantages of FSW, it has only found very limited commercial application to date due to many difficulties associated therewith. Modern FSW tools have relatively limited control systems. Such systems are primarily designed to simply join two identical flat members together by FSW. During joining, the FSW tool is moved through the joint at a constant speed, or conversely the workpieces are moved relative to the FSW tool at a constant speed. This constant speed control is the cause of many problems. First, the load that is required to be applied to the FSW tool in the direction of travel to ensure a constant speed varies throughout the welding process. Therefore, as it becomes more difficult to force the tool through the workpieces, due to any number of circumstances, a greater load is applied to the tool in the direction of travel, often resulting in a broken tool permanently solidified in the weld. Further, if the tool does not break, such widely variable loads result in premature tool wear. The load required to force the tool through the workpieces may vary due to temperature variations in the workpieces; thickness variations; as well as intended, or unintended, heat sinks resulting in a variable thermal profile over the length of the weld; among many other reasons. Forcing the tool through such regions of variable resistance at a constant speed often results in reduced weld quality due to inadequate mixing of the plasticized materials and reduced aesthetic quality due to overheating of the materials. Prior art attempts to deal with such issues have been focused on establishing multiple travel speeds along the length of the weld. For example, in joining workpieces that are eight feet in length; a first region, possibly by way of example, the first twelve inches, may be set to have a tool travel speed of one predetermined rate; a second region, perhaps the next seventy-two inches, having a tool travel speed of a second predetermined rate; and a third region, the remaining twelve inches, having a tool travel speed of a third predetermined rate. As a result of such crude stepped constant speed control, FSW has been limited to welds of simple travel paths on relatively simple components, thereby preventing widespread use in the material joining arts. In particular, these limitations have restricted the use of FSW on components having complex properties.
The instant invention addresses many of the shortcomings of the prior art and allows for previously unavailable benefits. A method for travel axis load control during friction stir welding has long been needed. The method and apparatus of the present invention is designed to overcome the travel speed and load control limitations of the prior art. Additionally, the method and apparatus do not introduce limitations into the FSW process and opens up the application of FSW to a wide variety of applications which were previously uneconomical. Further, the method and apparatus may adjust the load on the FSW tool in the direction of travel at a controlled rate to achieve an optimum thermal profile during welding. Alternatively, the method and apparatus may adjust or limit the load on the FSW tool based upon characteristics of the tool material, rather than on properties of the material being welded.